This invention relates generally to VAR generators and it relates in particular to high-speed circuits for ascertaining the firing angles in static VAR generators.
Static VAR generators compensate for the effects of highly-reactive loads such as arc furnaces on the terminal variables of AC supply systems. The static VAR generator or flicker compensator as it is sometimes called, includes a control circuit which measures the magnitudes of the arc furnace currents in consecutive half cycles of supply voltage. In some cases, arc furnace power may be measured rather than current. Regardless of which parameters are measured, the information is used to compute necessary compensation currents in terms of subsequently required firing angle signals for the thyristors of the static VAR generator. The VAR generator output current can only be adjusted once during each half cycle per phase. Consequently, the response time of the flicker compensator and thus its effectiveness for flicker reduction is largely dependent upon the speed with which the arc furnace current is measured and converted to a firing angle. U.S. Pat. No. 3,597,518, entitled "Electric Arc Furnace Control" by R. W. Roberts, issued Aug. 3, 1971 teaches a control system for an electric arc furnace which is designed to maintain constant arc peaks. The system is designed to respond quickly to large impedance errors, is insensitive to short-term impedance fluctuations, and at the same time is rarely highly sensitive to continuing small errors. U.S. Pat. No. 3,728,516, entitled "Welding Power Source" by A. D. Daspit, issued Apr. 17, 1973, teaches the concept of controlling SCR's in response to current and voltage applied to an electric welding device in which a capacitor is periodically semidischarged to a switching device. U.S. Pat. No. 3,936,727, issued Feb. 3, 1976 to F. W. Kelley, Jr. and G. R. E. Lezan teaches a compensation control device which determines the firing time of a static switch in accordance with a magnitude of the reactive load current and teaches a regulating means which additionally controls the firing time to maintain the line current and voltage at a selected line location substantially in phase coincidence. U.S. Pat. No. 3,999,117, issued Dec. 21, 1976 to Gyugyi et al. teaches a static VAR generator and compensator where time delayed firing angles are calculated from integrating furnace load currents over prescribed intervals during real time to thus maintain balanced load current at specified phase angles which are usually zero in a three-phase electrical system. The latter utilizes phase-to-phase voltage as a reference. In the latter system, the peak value of the phase-to-phase voltage delineates between operational cycles. Because of this, in the previous patents, it was taught to make the calculations for a control cycle prior to that cycle. The calculations were made over a period of approximately 180.degree. and were ready for utilization in the control circuit at the beginning of the cycle as measured by the reference voltage being at its maximum value. In a later U.S. Pat. No. 4,000,455, issued Dec. 28, 1976 by Gyugyi et al, it was taught that the total computation time for determining the firing angle in each succeeding half cycle would take no longer than the length of time of the last complete half cycle immediately prior to the firing of the thyristor. In U.S. Pat. No. 4,068,159, by Gyugyi et al., it was determined that the computation time for determining the firing angle could actually extend into the half cycle in which the correction was to be applied. This could be done because of the knowledge that the amount of current necessary for compensation is such that the firing angle could not occur before a minimum firing angle of a predetermined amount which may for example be 10.degree.. Consequently, the inventors in the previous case allowed the calculation of the actual firing angle to continue into the half cycle in which compensation was actually required by an amount equal to the previously-described minimum angular amount. It would be advantageous if apparatus and method could be discovered which extended the computation time for determining the firing angle even further into the half cycle than the previously-determined fixed minimum firing angle and thus utilized the extended firing angle time to continue to calculate information about the firing angle. As paradoxical as the latter statement seems to be, apparatus has been invented which performs that function.